Formed composite parts are commonly used in applications, such as aircraft and vehicles, where light weight and high strength are desired. These applications typically utilize complex contoured parts or channels which must be formed and then cured. Historically, complex contoured composite structures have entailed extensive hand labor to form prior to curing. Typically, pre-impregnated composite fiber plies (“pre-pregs”) such as epoxy impregnated carbon fiber laminates are laid by hand over a shaped form or mandrel. Then the part is cured, often by heat curing. Alternately, dry fabric plies (“dry fabric”) may be laid-up, and then a bonding material is added. This results in a contoured part that matches the shape of the mandrel. However, manual lay-up of pre-preg plies or dry fabric is time-consuming.
An alternate forming method known as drape forming uses vacuum bagging. Drape forming has been used successfully to form composite parts where the parts being formed have only a few pre-preg plies. This method involves heating a flat laminate pre-preg composite blank or charge and forcing it around a mandrel with the use of a vacuum bag. However, this method has met with limited success on very thick laminates or more complex shapes. More complex shapes include beams of various shapes such as C, I, or L shapes, with long flange lengths, contours along their length, variable thicknesses, joggles or offsets. Composite parts which are thicker in some areas and thinner in others have “ply-drops” where plies end. This leaves the final cured part thicker in some areas and thinner in others. Long flange lengths add strength to composite members such as those used in aircraft structures. In many applications, the composite parts to be formed need to be contoured or have joggles or direction changes internal to the part.
Vacuum bag drape forming of such parts often results in wrinkling of the plies. Wrinkles occur because some laminate plies are in compression when bent or urged over the mandrel, and buckle when there is no constraint on the bending portion to prevent out-of-plane-buckling. Similarly, on long flange parts, slip resistance between the plies during bending becomes too great, and inner plies buckle. Buckling or wrinkling of the plies also occurs over tools or mandrels that are curved or contoured, or have joggles along their length. Even slight contours of a radius on the order of thousands of inches is enough to initiate wrinkles. As the composite pre-preg charge is bent over the mandrel, if the length of the flange is too long or slip resistance between the plies is too great, out-of-plane-buckling of the laminate will occur.
Current state-of-the-art drape forming techniques using vacuum bags have not been able to control the stress state and shear forces occurring during the composite forming process. As a result, complex contoured shapes are typically manufactured by ply-by-ply hand lay-up techniques. An improvement to vacuum bagging uses an inflated bag under the bending portions of composite charge as it is formed. This inflated bag progressively deflates as the vacuum bag forces the composite charge over the mandrel. This method has been found to slightly decrease out-of-plane buckling. However, hand forming of thick laminates and more complex shapes is still performed to minimize out-of-plane buckling.
Compression molding techniques also have been utilized to form composite pre-preg and dry fabric charges over a tool or a mandrel. However, such methods have encountered the same difficulties in preventing out-of-plane buckling of the laminate during the forming process. In compression molding, a female mold matching the forming mandrel is forced over the composite charge and the mandrel to form the charge.
FIG. 1 is a cross-sectional view of a prior art vacuum bag forming system for forming composite materials. A composite charge 20 is placed over a mandrel 10. It will be appreciated that the composite charge may be any suitable material for forming composite parts, including, without limitation, dry fabric or pre-preg plies. The mandrel 10 rests upon or is linked to a vacuum base 26. The vacuum base 26, mandrel 10, and composite charge 20 are covered by a vacuum bag 24. During forming of the composite charge 20 over the mandrel 10, the charge 20 is heated and air is evacuated from beneath the vacuum bag 24, This forms the overhanging portions 21 of the composite charge 20 that overhang the top of the mandrel 10. In this example, vacuum bagging is used to form the flanges of a C-shaped beam or spar. The laminate plies in the overhanging portion 21 of the composite charge 20 shear past one another as composite charge is formed by the vacuum bag 24 over the mandrel 10.
FIG. 2A illustrates the prior art method of compression molding a composite charge 20 over a mandrel 10. A composite charge 20 is placed over a forming tool or mandrel 10. A compression mold 30 is forced over the composite charge 20 and the mandrel 10, pressing the composite plies against the mandrel 10 and forming the part. FIGS. 2B and 2C show improved methods of compression molding. In FIG. 2B, a composite charge 20 is placed over a mandrel 10. A compression mold 30 with flexible tips 32 bends the composite charge 20 by being forced over the composite charge 20 and the mandrel 10. The flexible tips 32 at the corners of the mold 30 decrease out-of-plane buckling in the composite charge as it is formed, by smoothing the plies as they are formed over the mandrel 10.
FIG. 2C shows a further variation of prior art compression molding of a composite charge over a mandrel. In FIG. 2C, the composite charge 20 is placed over the mandrel 10. A compression mold 30 with forming bladders 34 is forced over the composite charge 20 and the mandrel 10 to form the composite part. The forming bladders 30 press downward and laterally against the bending portions of the composite charge thus decreasing out-of-plane buckling. In FIGS. 2A, 2B, and 2C, the laminate plies of the composite charge 20 overhanging the mandrel 10 shear past one another over the entire overhang or flange length during the forming process. This creates a tendency for out-of-plane buckling, especially with thick laminates, long flange lengths, contoured parts, joggles or parts with inflections.
FIGS. 3A, 3B, and 3C illustrate the large surface area where laminate plies shear past one another during forming of a composite charge 20 over a mandrel 10, utilizing the prior art methods of vacuum bagging or simple compression molding illustrated in FIG. 1 and FIGS. 2A, 2B, and 2C. In FIG. 3A, a flat composite charge 20 is placed over the mandrel 10. In FIG. 3B, as bending of the composite charge 20 occurs, a shear zone 22 exists where the laminate plies shear past one another. This inter-ply shear zone encompasses the entire overhang length or flange length of the part being formed. The magnitude of the shearing increases towards the edge of the flange.
In FIG. 3, shearing between the laminate plies in the shear zone 22 continues as the composite charge 20 is forced down over the mandrel 10. Shearing within the shear zone 22 results in out-of-plane buckling of laminate plies. Under prior art methods of vacuum bagging and compression molding, inner plies of the composite charge laid against the mandrel, are in compression from shearing against the outer plies as the composite charge 20 is formed over the mandrel 10. This is shown in prior art FIGS. 3B, and 3C, where the entire flange area 22 has slipping between the plies.
Therefore, an unmet need exists for a composite forming method and system which forms thick laminate charges and parts with contours, joggles, or long flanges, without out-of-plane buckling of the laminate plies.